1. Field of the Invention
The present invention relates to a recording medium, and, more particularly, to a hologram recording medium on which data is recorded as interference fringes via the use of an object beam and a reference beam. The present invention also relates to a recording apparatus and a reproducing apparatus to record/reproduce data on a hologram recording medium.
2. Description of the Related Art
A hologram recording method in which data is recorded on a recording medium by using holography is performed by simultaneously irradiating an object beam and a reference beam. The object beam has image data to be recorded in a hologram recording medium. The simultaneous irradiation of these beams causes interference fringes to be written onto the hologram recording medium. When reproducing data from a hologram recording medium, the same reference beam as used in the recording operation is radiated onto the hologram recording medium and the image data recorded on the hologram recording medium is reproduced by detecting a diffraction that is caused by the interference fringes.
In another hologram recording method, the interference fringes discussed above may be three-dimensionally recorded by additionally utilizing a thickness direction of the hologram recording medium. The hologram recording medium then has a remarkably increased recording capacity compared to a surface two-dimensional recording medium, such as a CD or a DVD, due to the hologram recording medium having an ability to re-record overlapping two-dimensional image data on the same region.
A hologram recording and/or reproducing apparatus is a type of interference system using interference between an object beam and a reference beam. In such an interference system, irradiating two beams stably is difficult. This difficulty has led to various studies aimed at overcoming this drawback. One solution employs a fine step measuring apparatus in which an optical system, referred to as a common optical path-type interference system, has been used. In such an optical system, two beams are transferred along the same optical path. Thus, changes in optical path due to disturbances, such as vibration or air turbulence, affect both beams equally and thus are cancelled. Accordingly, a stable apparatus, which is not affected by changes in optical path, may be realized.
One example of such an optical system which is widely used is a Normarski interference system or a Normarski microscope. In addition, in a common optical path-type interference system, since two beams propagate through the same optical path, the optical system may be formed with a simple structure and a small size.
FIG. 12 is a perspective view of an optical system in a conventional hologram recording and/or reproducing apparatus, an example of which is disclosed in U.S. Pat. No. 6,108,110. Referring to FIG. 12, a spatial light modulator (SLM) is arranged around the center of an optical system to display recorded data, which is converted into a two-dimensional digital image. In the SLM shown in FIG. 12, the light beam's intensity is modulated to carry information and serve as an object beam.
As shown in FIG. 12, a reference beam is arranged at a position outside of the object beam. The object beam and the reference beam are irradiated in a hologram recording medium to record interference fringes. The hologram recording medium is rotated during recording so that data from the object beam is multiplex-recorded on the hologram recording medium. When reproducing data from the hologram recording medium, a beam output from the SLM is blocked and the reference beam is radiated onto the interference fringes for a two-dimensional image sensor, such as a CCD, to receive image data reproduced from the interference fringes and to reproduce data.
Since a large amount of image data is multiplex-recorded on the same region of a hologram recording medium using this hologram recording method, the capacity of the hologram recording medium may be increased by increasing a thickness thereof. However, the recording capacity of the hologram recording medium is, in fact, limited for various reasons, and if a beam emitted by an optical device is scattered, for example, by an astigmatic lens or the hologram recording medium itself, a serious problem results.
In general, as the number of multiplex images (i.e., the number of the interference fringes), increases, the diffraction efficiency of a reproduction beam, which is diffracted at each of the interference fringes, rapidly drops. On the other hand, when a beam is radiated onto an optical device, such as a lens, or a hologram recording medium, a scattered beam is generated due to the roughness of the surface or unevenness of the material of the hologram recording medium. Preventing beam scattering is therefore substantially impossible. Thus, an image sensor picks up a scattered beam mixed with a reference beam. In addition, the scattered beam is optical noise and interferes with detection of a reproduction beam whose diffraction efficiency is low. Accordingly, the maximum recording capacity of a hologram recording medium is determined by the ratio of the optical intensity of a reproduction beam to the optical intensity of a scattered beam, in other words, an S/N ratio.
Since the reference beam and the object beam are transferred along the same optical path in the conventional system of FIG. 12, the apparatus may be stable and small. However, because of the common optical path, the scattered beam may be easily input to an image sensor. Accordingly, the common optical path-type hologram recording and/or reproducing optical system of FIG. 12 cannot substantially increase a recording capacity.
FIG. 10 is a graph illustrating the relationship between numerical aperture (NA) of an objective lens and number of multiplexed holograms, when 0.2 tera-bytes, 0.5 tera-bytes, and 1 tera-bytes of data are to be recorded on a hologram recording medium having the same recording area as a CD. When the NA of a conventional objective lens is determined to be 0.5, the numbers of multiplex holograms are 400, 1,000, and 2,000 for 0.2 tera-bytes, 0.5 tera-bytes, and 1 tera-bytes of data, respectively.
FIG. 11 is a graph illustrating the relationship between diffraction efficiency and number of multiplexed holograms. Diffraction efficiency (η) is calculated by dividing an M number (M#), which denotes the characteristic of a recording material, by a hologram multiplexing number (M) and squaring the result. For example, when the M number of a conventional hologram recording material is 5 and the hologram multiplex numbers are 1,000 and 2,000, the diffraction efficiency is 2.5×10−5 and 6.3×10−6, respectively.
FIG. 9 is a graph illustrating the relationship between amount of scattered light, which is measured in a conventional hologram disk storage system of FIG. 12, and diffraction efficiency (η). Referring to FIG. 9, the minimum diffraction efficiency is 1×10−2 due to scattered beams in the conventional system, and a diffraction efficiency as small as 1×10−5, which is required to achieve tera-byte recording capacity, cannot be detected.
In the conventional system of FIG. 12, the object beam is located at the center of the optical system and the reference beam surrounds the object beam such that the two beams are separated spatially. However, in this configuration, a scattered beam may propagate in every direction. Thus, eliminating the scattered beam by simply spatially separating the reference beam and the object beam is difficult.